The present invention relates to a thin, forged magnesium alloy casing suitable as light, strong casings for small electronic appliances and media and a method for producing such a thin forged casing.
Because magnesium has the smallest specific gravity of 1.8 among metal materials put into practical use at present, magnesium alloys are finding wide expectations and applications as light, strong materials alternative to aluminum having a specific gravity of 2.7 and it alloys. Magnesium alloys may be used for parts of aircraft and spacecraft, land transportation equipment, cargo equipment, industrial machines and tools, electronic equipment, telecommunications equipment, agricultural machines, mining machines, office equipment, optical equipment, sports gear, etc.
The magnesium alloys are, however, much poorer in plastic working than aluminum alloys. Accordingly, the magnesium alloys are usually provided as die-castings at present. To improve castability and mechanical strength, magnesium is alloyed with aluminum, zinc, etc. Zirconium may be added to provide strength and toughness, and manganese may be added to make the crystal grains of the magnesium alloys finer. Also, rare earth elements and silver may be added to provide heat resistance.
However, magnesium alloy castings are limited to relatively thick products, because it is extremely difficult to cast magnesium alloys into thin products. In addition, casting defects such as pores and inclusions such as oxides, which are inevitable in casting, may be contained in the magnesium alloy castings and appear on the surface thereof. The casting defects and the inclusions deteriorate the mechanical strength of the magnesium alloy castings, and if they appear on the surface, they adversely affect the corrosion resistance and surface appearance of the castings.
Recently proposed and attracting attention is a so-called semi-solid method for forming magnesium alloy members in a temperature range in which a solid phase and a liquid phase coexist, by utilizing an injection technique. Products obtained by this forming method have fine crystal structures free from dendrites existing in usual castings, and also have higher density with fewer pores than die-castings, whereby they can be subjected to a heat treatment. This method can produce magnesium alloy members as thin as 1.5 mm or less. Nevertheless, the semi-solid, forming method is disadvantageous in that magnesium alloy members produced thereby are not necessarily free from defects and oxide inclusions inside and on the surface. With defects and oxide inclusions, good surface conditions such as appearance and corrosion resistance cannot be obtained.
Another method for forming thin magnesium alloy products is a drawing method. The drawing method comprises casting a magnesium alloy into an ingot; forging the ingot to remove or reduce defects and segregation; cutting or rolling the forged product to a proper length or thickness to form a thin plate; and drawing the thin plate to a desired shape. The drawing method is disclosed in Japanese Patent Laid-Open Nos. 6-55230 and 6-328155, Summary of the 89.sup.th Autumn Convention of the Light Alloys Association in 1995, pp. 179-180, etc.
Japanese Patent Laid-Open No. 6-55230 discloses that the deep drawing of a thin magnesium alloy plate can be carried out with a die with a punch and a flange portion heated to a surface temperature of 175-500.degree. C. In the Summary of the 89.sup.th Autumn Convention, a 1-mm-thick disc plate made of a magnesium alloy (AZ31) having a diameter of 60-65 mm is subjected to deep drawing with a punch having a radius of 40 mm and a shoulder radius of 12 mm and a die having a cavity having an inner diameter of 43 mm and a shoulder radius of 8 mm, at a blank pressure of 1000 kgf.
The deep drawing method, however, is only applicable to products having smooth surfaces, failing to provide products with projections. In addition, a smaller die shoulder radius than the above would cause cracking in the resultant products at inner bottom edges and corners, failing to provide products with sharp bottom edges and corners.
Because electronic circuits and elements are highly integrated and made denser recently, miniaturization and weight reduction are widely pursued in many applications such as mobile telecommunications gear such as cellular phones, note-type or mobile personal computers, electronic recording media such as compact disks, minidisks, etc. Casings for these appliances and media are mostly made of aluminum alloys at present, though further weight reduction is desired while keeping mechanical strength equivalent to or more than that of aluminum alloys. Magnesium alloys are promising because of their small specific gravity and high mechanical strength, if they can be forged into thin casings with sharp bottom edges, comers and projections.
Japanese Patent Laid-Open No. 6-172949 discloses a magnesium alloy part such as an automobile wheel, etc., and a forging method for producing such a magnesium alloy part. This forging method comprises (a) forging a magnesium alloy casting at a temperature of 300-420.degree. C. to form a forged part having an average crystal size of 100 .mu.m or less; and (b) subjecting the forged part to a T.sub.6 heat treatment comprising a solution treatment and an aging treatment. The forged part is subjected to finish working such as spinning and rolling. In a specific example, the above forging step (a) is carried out under the conditions that the magnesium alloy casting is heated at 400.degree. C., the die is heated at 250.degree. C., and the forging speed is 10 mm/sec. With an average crystal size of 100 .mu.m or less, the forged magnesium alloy has improved corrosion resistance and mechanical strength.
The technology proposed by Japanese Patent Laid-Open No. 6-172949 is, however, aimed at large, thick parts such as automobile wheels, etc., not coping with difficulty in forging extremely thin products with sharp bottom edges, corners and projections. It also requires the T.sub.6 heat treatment that takes a long period of time. If the technology of Japanese Patent Laid-Open No. 6-172949 is applied to forged casings of magnesium alloys, the resultant forged casings would not be able to be made as thin as 1.5 mm or less with sharp bottom edges, corners and projections, because the die at 250.degree. C. cools the magnesium alloy body too low to achieve smooth plastic flow (metal flow) of magnesium alloys during the forging.